This invention relates to improvements in conveyor systems of the type disclosed in Dahlgren U.S. Pat. No. 3,644,261, entitled "Straight Feed Press" and Dahlgren U.S. Pat. No. 3,847,079, entitled "Method of Printing Sheets". The disclosure of each of the aforementioned patents is incorporated herein by reference in its entirety for all purposes.
The aforementioned patents disclosed a sheet-fed printing press which incorporated a straight through and continuous sheet transfer principle, similar to the feeding style of a web-fed printing press, whereby sheets were grasped by gripper bars carried by flexible steel tapes or bands which extended around drive wheels for moving the sheets through a plurality of printing towers.
In printing it is critical that registration of the sheet to each printing cylinder be precisely maintained so that dots on each sheet precisely correspond to a corresponding dot on the other sheets. In single color printing, it is also critical that registration be maintained because sheets are often passed through a printing press more than one time to apply additional colors.
In the conveyor system described in the aforementioned patents, flexible steel tapes were driven by circular wheels mounted adjacent opposite ends of the printing press, with the band having indexing pins mounted therein. This created two major problems which tended to hamper marketability of the system.
First, the expansion of various materials are dependent upon the temperature, although for design purposes constant mean values are usually employed for design purposes. If lengths, areas, and volumes are at a standard temperature, the approximate change in dimensions of the material will be considered to be a function of the change in temperature. At best, such design criteria provides only approximate dimensions and resultant inaccuracy in speed and location of parts. Even though all components of the printing press may be constructed of materials having an identical co-efficient of thermal expansion, the dimensions of various components of the system may change non-uniformly which results in further variation of the speed and location of various parts of the system which is detrimental to registration of the press.
Second, even if thermal expansion is ignored, it is virtually impossible to construct and drive through circular members to maintain absolute registration. The Greek letter .pi. pronounced "pi", is the ratio of the circumference of any circle to its diameter and stands for the number by which the diameter of a circle must be multiplied to obtain the circumference. Thus, the circumference of a circle is equal to .pi. times the diameter of the circle.
The number .pi. cannot be exactly expressed as a decimal. The common values used to express .pi. include 22 divided by 7, 3.14, 3.1416, and 3,14159. Rounded off to twenty decimal places, .pi. is approximately equal to 3.14159265358979323846.
Thus, the technical problem exists of obtaining absolute accuracy in manufacturing circular members and of maintaining near absolute accuracy of components and replacement parts under varying conditions of temperature, speed, acceleration and other operating conditions.
Various devices have been devised heretofore for adjusting the tension in a web and the distance a web travels between adjacent printing cylinders in an effort to maintain registration in web-fed printing presses. However, such devices are not readily adaptable for sheet-fed printing presses of the type disclosed in the aforementioned patents because the web is generally routed along a serpentine path to compensate for errors in registration between colors.